The invention is directed toward a photon detector. More particularly, the invention is directed to an improved photon detector which uses an activated lanthanide beryllate, such as activated lanthanum beryllate, as a scintillation material. The term "photon" as used herein includes both gamma rays and X-rays.
Photon detectors are used in a wide variety of applications, for example, oil exploration and medicine. Photon detectors usually employ specially grown scintillation crystals which produce a flash of visible or near-visible photons when a high-energy photon interacts with electrons in the crystal. This flash of visible or near-visible photons is then sensed, for example, in a photomultiplier tube (PMT) which is adjacent to the scintillation crystal and the PMT produces an electrical signal indicative of the flash intensity. The electrical signals are then analyzed.
For example, in subsurface surveying applications a probe, or sonde, is drawn through a borehole and a photon detector in the sonde measures the energy and/or intensity of radiation returning from the strata. Oil, gas, water, and various other geological formations possess distinctive radiation signatures that permit identification of the make-up of the strata. Analysis of the time and energy spectrum distributions of detected high-energy photons thus provides information about subsurface conditions.
General background information on subsurface surveying is provided in U.S. Pat. No. 5,008,067, issued to John B. Czirr on Apr. 16, 1991 and entitled "Method of Downhole Neutron Monitoring" and U.S. Pat. No. 5,313,504, issued to John B. Czirr on May 17, 1994 and entitled "Neutron and Photon Monitor for Subsurface Surveying" The entire contents of these documents are incorporated herein by reference.
Two well known scintillation materials are bismuth germanate (BGO) and cerium fluoride (CeF.sub.3). These two materials have certain drawbacks, however, relating to their decay time, ability to operate effectively at high temperatures, and light emission efficiency. For example, bismuth germanate does not scintillate at the high temperatures frequently encountered downhole.